Certain pharmaceuticals may be delivered to the nose and/or lungs of a patient by inhalation, using an inhaler device of which there are several known types. In some of these devices, the drug (or a formulation containing the drug, for instance together with a pharmaceutically acceptable excipient such as lactose) is suspended in particulate form in a fluid vehicle, which acts to transport the drug in a suitably disperse state towards the intended site of administration. The vehicle may be a pressurized propellant fluid if the drug is to be delivered in aerosolized form. “Metered dose inhalers” (MDIs) may for example be used to effect such delivery, for instance those used to dispense budesonide (Pulmicort™, AstraZeneca), salbutamol (Ventolin™, Glaxo SmithKline and Proventil™ HFA, Schering Plough), salmeterol xinafoate (Serevent™, Glaxo SmithKline) and fluticasone (Flovent™, Glaxo SmithKline).
Typical propellant fluids include hydrofluoroalkanes such as 1,1,1,2-tetrafluoroethane (available as HFA 134a), 1,1,1,2,3,3,3-heptafluoropropane (available as HFA 227ea) and 1,1,2,2,3-pentafluoropropane.
The particulate drug must be suspended as uniformly as possible in the fluid vehicle. This is usually achieved by shaking the inhalation device prior to dispensing a dose of the drug. It is clearly desirable that the drug remains suspended in the vehicle for a sufficient length of time after shaking to allow it to reach the intended site of administration. However, particulate drug/propellant suspensions tend only to be stable for limited periods of time. Where the drug is more dense than the propellant, the tendency is for it to “settle” or “flocculate”, ie, to fall out of suspension. Where it is less dense than the propellant, the drug tends to “cream” or float towards the top of the propellant volume. This can reduce the efficiency and therefore also accuracy of drug dosage delivery. Often dispersion enhancing agents such as surfactants need to be added to the drug/propellant mixture to achieve and sustain a suitably uniform suspension.
Even using such techniques, it has typically proved difficult to prepare inhalable suspensions which are stable during normal storage periods and conditions and which give uniform dosing throughout the useful life of the average inhaler.
It has also been proposed to use hollow, or at least partially fluid containing, particles in MDI formulations in order to obtain improved dispersibility—see for instance the perforated microstructures described in U.S. Pat. No. 6,309,623 and the hollow microspheres disclosed in WO-97/36574, both suggested for use in inhalers.
Particulate active substances, such as drugs, may be produced by a variety of known methods, including for example crystallisation from solution, anti-solvent precipitation from solution, milling, micronisation, spray drying, freeze drying or combinations of such processes. Also known are particle formation processes which make use of supercritical or near-critical fluids, either as solvents for the substance of interest—as in the process known as RESS (Rapid Expansion of Supercritical Solution—see Tom & Debenedetti, J. Aerosol. Sci., 22 (5), 555-584 (1991))—or as anti-solvents to cause the substance to precipitate from another solution—as in the process known as GAS (Gas Anti-Solvent) precipitation (see Gallagher et al, ACS Symp. Ser., 406, p334 (1989)).
In general, however, known processes for producing inhalable drugs yield particles which perform poorly in propellant fluids, ie, they exhibit poor flocculation behavior. For many known particulate drugs, the tendency to flocculate can be a severe problem, with significant settling occurring within less than a minute of shaking the suspension and thus often before a dose of the drug has been successfully dispensed or at least before it has reached its target site of administration.
It would therefore be desirable to provide particulate drugs, and indeed other active substances which may need to be delivered in suspension in fluid vehicles, which have improved flocculation behavior in such vehicles.